A digital signal controller (DSC) includes functionality of a microcontroller and a digital signal processors (DSPs). Similar to a microcontroller, a typical DSC includes controlled peripherals such as pulse wide modulation (PWM) modules and timers. Similar to DSPs, a typical DSC includes single-cycle multiply-accumulate (MAC) units, barrel shifters, and large accumulators. Not all vendors have adopted the term DSC, some continue to use the term microcontroller although such microcontrollers include the added functionality typically associated with a DSP. DSCs are used in a wide range of applications including, but not limited to, power conversion, motor control, and sensor processing applications.
PWM modules are commonly used for controlling power delivered to a load, such as an electrical device. A PWM module generates a PWM signal that drives a switch. The average value of voltage (and current) fed to the load is controlled by turning the switch ON and OFF. The longer the switch is ON compared to OFF, the higher the power supplied to the load. The duty cycle describes the proportion of ON time to the total period of the PWM signal. A low duty cycle corresponds to low power, because the power is OFF for most of the time. Duty cycle is expressed in percent, 100% being fully ON. PWM works well with the ON/OFF nature of digital controls, which can easily set the needed duty cycle.
DSC vendors typically design the DSC to include a general purpose digital PWM module. It is left to the device manufacturer into which the DSC is installed to specifically configure the digital PWM module. The DSC is configured to execute a control algorithm, which in part provides an input to the digital PWM for controlling the duty cycle. In the case where the DSC is used in a switching mode power converter, controlling the duty cycle of the digital PWM enables regulation of the power converter output voltage.
It is a common application for a digital PWM to be configured with multiple channels for interleaving operation. In an exemplary application, a DSC is included in a switching mode power converter having digital PWM module with two channels. The digital PWM is configured to operate in one of two operation modes, an independent operation mode and a complementary operation mode. In the independent operation mode, the two channels are operated independently of each other, for example the duty cycle for each channel is independent. In the complementary operation mode, each of the two channels is correlated as a complement of each other where the PWM signal for the second channel is phase shifted 180 degrees from the PWM signal for the first channel. For example, if the first channel is operating at 10% duty cycle, then the second channel is operating at 90% duty cycle. As such, in the complementary operation mode, the channels can not operate independently. Instead, the duty cycle of the second channel is restricted to the complement of the first channel duty cycle. The mode is set in firmware that controls the DSC.
The complementary operation mode is the common operation mode used to achieve the desired pulse width for power factor correction (PFC) interleaving operation. However, DSCs provided by certain vendors suffer from random pulses during device power up when configured for complementary operation mode. Power up is that period of time immediately following the device being turned ON and the device stabilizing at normal operating conditions. In the case of power supplies, random pulses output from the digital PWM may damage or destroy a PFC front-end circuit due to saturation of the magnetic component within the PFC front-end circuit.